1. Field of the Invention
The present invention relates to a laser apparatus and, more particularly, to a metal vapor laser apparatus using, e.g., copper as a laser medium.
2. Description of the Related Art
A metal vapor laser has attracted attention as a light source used in uranium enrichment. In uranium enrichment, .sup.235.sub.92 U serving as a fuel for atomic fission nuclear electric power generation is separated from natural uranium and enriched. Since an abundance of .sup.235.sub.92 U is 0.071% in the natural uranium, .sup.235.sub.92 U must be enriched to be about 3% in order to be used as a nuclear fuel. For this purpose, in an uranium enrichment atom method using a laser beam, only .sup.235.sub.92 U is excited and ionized by a dye laser or the like and separated by an electrode applied with a voltage. In this case, the dye laser is a special laser in which in order to oscillate a dye laser beam, another type of laser beam is used. That is, in order to excite a laser medium of the dye laser, another type of laser beam is used. An example of the laser used for oscillating the dye laser beam is a metal vapor laser. An example of the metal vapor laser for dye laser excitation is a copper vapor laser.
FIG. 1 shows a conventional metal vapor laser apparatus. The apparatus has discharge tube 102, the interior of which is kept airtight. Tube 102 consists of central portion 103 and two end portions 105 and 107. Central portion 103 of tube 102 has heat insulating member 106. Cylindrical airtight vessel 104 is arranged outside member 106. Vessel 104 is concentrically arranged. Cylindrical core tube 108 is located inside member 106. A metal vapor source, e.g., a plurality of grains of copper materials 110 are located inside tube 108. Substantially annular cathode and anode electrodes 112 and 114 are located at both ends of tube 108 and member 106. Each of electrodes 112 and 114 has an L-shaped section along a plane including an optical axis. Electrodes 112 and 114 are connected to electric wires 116 extending from power source unit 118. In order to reliably insulate electrodes 112 and 114, annular high-voltage insulating member 120 is formed to be in contact with electrode 112 and vessel 104. Laser beam transmission windows 122 and 125 are arranged at end portions 105 and 107 of discharge tube 102, respectively. Sealing members 123 are arranged between window 122 and portion 105, and between window 125 and portion 107. Windows 122 and 125 are mounted at equal distances from electrodes 112 and 114, respectively. In this case, windows 122 and 125 are arranged to form a Brewster angle with respect to the optical axis. High reflecting mirror 124 is arranged next to one window to reflect a laser beam transmitted through window 122, and output mirror 126 is arranged next to the other window. Cooling pipe 128 is wound around the outer surface of tube 102. Gas supply unit 130 is located at end portion 105 of tube 102, and vacuum pump 132 is located at end portion 107 thereof.
The conventional metal vapor laser apparatus having the above arrangement oscillates a laser beam as follows. First, pump 132 is activated to evacuate tube 102 to obtain a negative pressure therein. A buffer gas, e.g., Ne gas is supplied at a pressure of about 10 to 20 Torr from gas supply unit 130 to tube 102. Then, electrical discharge is generated between electrodes 112 and 114 by, e.g., a pulse voltage applied from lower source unit 118. Copper materials 110 as a metal vapor source are heated by this discharge. Heated copper materials 110 are partially evaporated to be metal atoms and diffused in tube 102. In the above state, the buffer gas is ionized or excited by discharge. When the buffer gas collides against the metal atoms, energy is transferred to the metal atoms to excite them. The excited metal atoms transit to generate a laser beam. This laser beam is resonated and amplified between mirrors 124 and 126. As a result, a laser beam is emitted from mirror 126.
In the metal vapor laser apparatus which oscillates a laser beam as described above, core tube 108 is heated by electrical discharge between electrodes 112 and 114. The temperature of tube 108 is heated to a thousand and several hundred degrees. Metal atoms evaporated in this state are partially, positively charged by discharge. The positively charged metal atoms move toward cathode electrode 112. Sealing members 123 are used, however, in order to airtightly mount windows 122 and 125 on tube 102. In this case, a material of sealing members 123 must be kept at a temperature of 200.degree. C. or less. For this reason, the metal vapor evaporated in high-temperature core tube 108 and moving toward cathode electrode 112 tends to collide against window 122 located along its moving direction. Since window 122 is at a relatively low temperature, a metal vapor or an impurity is adhered and solidified on window 122. In addition, a temperature distribution in the core tube is not uniform but a highest temperature portion is offset to the cathode side. For this reason, the density of the metal vapor is higher at the cathode side, and therefore the metal vapor or the like tends to be adhered on window 22. As a result, the transmission windows are contaminated by the metal vapor and their light transmittivity is reduced over time. If the transmittivity of the windows is reduced, an oscillation efficiency of a laser beam is reduced This oscillation efficiency reduction poses a problem of reduction in a service life of the metal vapor laser apparatus.
In order to solve the above problem, the two transmission windows arranged at the both ends of the discharge tube may be separated sufficiently from the corresponding electrodes. In this method, however, the overall length of the discharge tube is increased to undesirably enlarge the apparatus. In addition, when a pulse width of the pulse voltage to be applied to the electrodes is about 20 ns, the number of times of oscillation of the laser beam between the mirrors is reduced. Therefore, problems such as degradation in a short pulse laser beam quality or ASE (Amplified Spontaneous Emission) are posed.